Magnetic sound recorder



'Feb. 4, 1941. E, C LER 2,230,913 MAGNE TIC SOUND RECORDER Patented Feb.4, 1941 UNITED STATES MAGNETIC SOUND RECORDER Eduard Schiller,Berlin-Johannisthal, Germany,

assignor to Licentia Patent-Verwaltungs-Gesellschaft, Berlin, GermanyApplication August '17, 1939, Serial No. 290,389 In Germany March 3,1936 6 Claims. (01479-1002) This invention relates to magnetic soundrecorders, and. particularly to telegraphones or devices of the type inwhich the sound is recorded by selectively magnetizing successiveportions 5 of a traveling wire or band in accordance with the magnitudeand frequency of the sound waves;

The practice has been to pass the sound track or magnetic hand throughthe field of a strong magnet as it approaches the recording electro- 1magnets, thereby erasing any previous history before a record is enteredon the moving band. The magnets and electromagnets previously employedfor this purpose have not been entirely satisfactory as the magneticfield between the two poles of the magnet varied greatly both as tointensity and direction. A part of the magnetic field would extendlongitudinally of the sound carrier but the adjoining leakage fieldextended in various directions and was of such strength that themagnetization that ultimately remained on the sound carrier deviatedconsiderably from I the desired longitudinal direction.

An object of this invention is to provide a magnetic sound recorderhaving erasing magnets with pole pieces of such design that the activestray field extends in the longitudinal direction of the sound carrier.An object is to provide an electromagnet for removing any prior recordsfrom a magnetic sound carrier as it approaches the recording head; thecore of the electromagnet being approximately circular with a smallradial gap across which the sound carrier travels in a directiontangential to the core, the pole piece at the departure side of the corehaving a longer active surface than the pole piece at the advance sideof the core. A further object is to provide a magnetic system forconditioning a magnetic sound carrier as it approaches the recordinghead, the magnetic system including an approximately circular magnethaving a short gap across which the sound carrier passes and a trailingpole piece at which the leakage fiux extends longitudinally of the soundcarrier and in a direction opposite that of the fiux at the gap of themagnet Another object 55 invention will be apparent from the followingspecification when taken with the accompanying drawing in which: i

Fig. 1 is a schematic view of an erasing electromagnet that constitutesone embodiment of the present invention; 5

Fig. 2 is a schematic side elevation of another construction;

Fig. 3 is a fragmentary plan view of the same; and

Fig. 4 is a curve sheet showing the magnetiza- 10 tion of the soundcarrier at difierent points along its path of travel across the erasingmagnet and the recording head.

In Fig. 1, the core I is approximately circular and carries adirect-current magnetizing wind- 15 ing 2 that establishes a strongmagnetic field at the gap 3 across which the magnetizable sound carrier4' travels in a tangential direction. The sound carrier moves from leftto right as indicated by the arrow :1: and the pole piece at the 20right or departure side of the core I is spaced from the sound carrier 4by a greater distance than is the pole piece at the left or approachside of the core, the terms departure and approach having reference tothe travel of the 25 hand t. The modification of the core I from a truecircular shape, such as was previously used, has been greatlyexaggerated in Fig. 1 as the desired distribution of the magnetic fieldcan be obtained when one-pole piece is bent inwardly 30 of the other byas little as'about 1 mm. A block or strip of non-magnetic material-5 maybe located in the space between the inturned pole piece and the band 4to provide a supporting table for the moving band.- 35

The broken line vectors a indicate the magnetic field that extendslongitudinally of the band t at the gap 3. The particular shape of themagnetic core extends the field along the record carrier and the activepolar surface of an the pole piece at the departure side of the core issubstantially longer than the active polar surface at the approach side.

In the construction shown in Figs. 2 and 3, the core l' carries adirect-current winding 2 for 4,5 establishing a strong magnetic fieldacross the gap 3 between the pole pieces that are of difi'erenteffective, lengths, as measured in the direction of travel of the recordcarrier 5. Both pole pieces have plane surfaces along which the recordcarrier travels, and the pole piece at the departure side of the core isextended beyond the adjacent leg of the core to form a polar tip 8 atwhich the leakage fiux, indicated by the dot-' ted line vectors 6, isapproximately longitudiml of the record carrier but in a directionoppositegto thatof the field a at the gap 3. MA polsrizillng field c, inthe same direction as the satura as magnetic field a. is established atthe narrow slot I oi the recording magnet I by the direct-currentwinding 9. The alternating field corresponding to the sound or signal tobe recorded is produced by current flow in winding Ilionthe core 8.

The method of operation may be best understood irom a consideration ofthe typical hysteresis curve of Fig. 4. Each magnetic particle of therecord carrier passes the gap 3 for which the magnetic field intensityis represented in Fig. 4

by the vector quantity a. The flux density in the ferromagnetic band 4rises to the value P1 along some line such as P1 as the magneticparticle approaches the gap 3, the exact shape oi this part of the curvedepending upon the prior magnetic condition of the particle. Theintensity of field a. is sumciently great to produce saturation andtherewith the extinction of the prior magnetic history ofthe particle.The field intensity is zero at some point along the pole piece beyondthe gap 3 and the induction falls to a value P: that is determined bythe retentivity of the carrier 4. The particle then enters the field bat the polar tip 6 where the flux density of the carrier is representedby point P3 of the curve. The flux density has a value corresponding topoint P4 after it leaves the field b, and then rises to a value P underthe influence of the unidirectional polarizingfleld c that issuperimposed upon the alternating field d of the signal at the recordinghead. The point P5 is the operating point on the magnetization curveabout which the induction is varied by the alternating field d.

, It has been demonstrated that telegraphone I record carriers treatedby the described erasing magnetic systems give a higher sound output atthe reproducer, for equal intensity input signal currents, than therecord carriers from which the prior history was removed by thepreviously known erasing magnets. The record carriers exhibited a lowernoise level when pretreated by the new erasing magnets and thus permit.higher amplification of the reproduced 1 signals. t

ing a longer active suriace than the pole piece at the advance side 01the core.

2. In magnetic sound recording apparatus, a magnet as claimed in claim1, wherein the pole piece at thedeparture side of the core is moredistant from the path of travel oi the record strip than is the otherpole piece, and a strip of non- 'magnetic material is located betweenthe path of the record strip and said pole piece at the departure sideof the core. 7

3. In magnetic sound recording apparatus, the invention as claimed inclaim 1, wherein the pole piece at the departure side of the coreextends along the path of travel of the record strip and terminates in apolar tip at which the direction of the leakage flux is opposite that ofthe flux at said radial gap.

4. In apparatus for the recording of signals on strips of magneticmaterial, a magnet for eliminating'prior recordings from a strip as itapproaches a recording unit; saidmagnet comprising an approximatelyannular core and a direct current winding on said core, said annularcore having an air gap over which the record strip travels, the coresection at the departure side of the gap being spaced from the recordstrip by a distance substantially greater than that between the core andrecord strip at the advance side of the air gap.

5. In magnetic sound recording apparatus, an erasing head comprising acore having pole pieces spaced apart by a gap across which the recordcarrier travels, and a direct current windingon the core, the pole pieceat the departure side of the core being extended along the recordcarrier path to form a leakage polar tip at which the direction of theleakage flux is opposite to the direction of the flux at the air gap.

6. In magnetic sound recording apparatus, th combination with meanscreating a strong magnetic field substantially longitudinally of amoving record strip, means acting upon a record strip after the strippasses beyond said .first means to establish a magnetic field of lessintensity and extending longitudinally of the strip in 9. directionopposite that of the first field, and a recording head for magnetizingsaid record strip after it passes from the second means, said recordinghead including a magnetic core and a winding on said core for connectionto a source of audio frequency energy, of polarizing means forestablishing a magnetic field longitudinally of the record strip and inthe same direction as the field of said first means, said polarizingmeans comprising a direct current winding on the magnetic core of therecording head.

EDUARD scHiiLLEn.

CERTIFICATE OF CORRECTION. Patent No. 2,250,915. February L 19in.

EDUARD SCHULLER.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,first column, line 55, claim 1, after the word "pieces" insert--spaced--; and that the said Letters Patent should be read with thiscorrection therein that the same may conform to the record of the casein the Patent Office.

Signed and sealed this 25th day of February, A. D. 19141.

7 i I Henry Van Arsdale, (Seal) y Y I Acting Commissioner of Patents.

